add padding_idx setter (#74946)

Author: zhiminzhang0830

python/paddle/nn/layer/common.py

@@ -1870,6 +1870,10 @@ def __init__(
     def padding_idx(self):
         return self._padding_idx
 
+    @padding_idx.setter
+    def padding_idx(self, value):
+        self._padding_idx = value
+
     @param_one_alias(["x", "input"])
     def forward(self, x: Tensor) -> Tensor:
         return F.embedding(

test/legacy_test/test_nn_dtype_device_bias.py

@@ -508,6 +508,9 @@ def test_padding_idx(self):
             layer = self.api(32, 16, padding_idx=2)
             assert layer._padding_idx == layer.padding_idx
 
+            layer.padding_idx = 5
+            assert layer._padding_idx == 5
+
 
 if __name__ == '__main__':
     unittest.main()